Organic light-emitting diode (OLED) technology holds significant promise as a display technology that is well suited to a broad range of applications. Self-emitting, OLED displays are advantaged over other display technologies, providing high luminance, good quality color, and relatively wide viewing angle. OLED display components are thin and lightweight, making them particularly adaptable for use with handheld components, such as cameras, cell phones, personal digital assistants (PDAs) and laptop computing devices.
The basic bottom-emitting OLED pixel 10 is constructed as shown in FIG. 1. An organic layer 12, typically fabricated as a stack of multiple thin organic layers, is sandwiched between a cathode 14 and a transparent anode 16, built onto a glass substrate 18. Organic layer 12 includes an electroluminescent layer (EL) that provides illumination when appropriate voltage is applied between anode 16 and cathode 14. Pixel 10 is formed in the overlap area between cathode 14 and anode 16. An OLED display is formed from an ordered spatial arrangement of individually addressable OLED pixels 10 arranged as an array, in successive rows and columns.
There are two basic types of OLED arrays, passive matrix and active matrix. Active matrix OLED displays integrate current control circuitry within the display itself, with separate control circuitry dedicated for each individual pixel element on the substrate for producing high-resolution color graphics at a high refresh rate. Passive matrix OLED displays, on the other hand, have current control circuitry that is only external to the display itself. Thus, passive matrix OLED displays are of simpler construction than are active matrix displays, and permit simpler, lower cost fabrication techniques.
The basic arrangement of a passive matrix OLED array 20 is shown in the simplified schematic of FIG. 2a. In array 20, each individually addressable OLED pixel 10 has an electroluminescent diode 11 connected between an anode line 26 (column) and a cathode line 24 (row). Each anode line 26 has a current source 22 that is switched ON to anode line 26 in order to illuminate pixel 10 in each column, according to image data. Cathode line 24 is commonly shared by each electroluminescent diode 11 in a row. A switch 30 for each cathode line 24 switches to ground to enable illumination of pixels 10 in each successive row, one row at a time, using a scanned row sequence. An electroluminescent diode 11 illuminates when its current source 22 is switched ON and its corresponding row switch 30 switches to ground. Otherwise, cathode lines 24 are typically switched to an intermediate voltage Vi. Electroluminescent diodes 11, whose cathode potential is at Vi, do not illuminate. Having its cathode line 24 at intermediate voltage Vi turns pixels 10 off for any row that is not being scanned, but maintains a potential on the row. This reduces the amount of power necessary to charge the parasitic capacitance of each row as it is addressed. Using this straightforward arrangement, passive matrix OLED array 20 can be constructed to have several thousand pixels 10, organized in a matrix of rows and columns. Control logic in a display apparatus (not shown) provides control of current source 22 for each column and of switch 30 for each row, making each OLED pixel 10 individually addressable, using control and timing techniques well known in the display component arts.
It must be emphasized that the above description and schematic of FIG. 2a provide a simplified explanation of the control mechanisms and composition of passive matrix OLED array 20. More detailed information on prior art passive matrix OLED arrays and array driver solutions can be found, for example, in U.S. Pat. No. 5,844,368 to Okuda et al. and in U.S. Pat. No. 6,594,606 Everitt.
While smaller OLED displays of several inches in diagonal have been successfully built, fabrication defects still present obstacles to the development of large area OLED displays of the passive matrix type. Defects can be due to dust or contamination during fabrication, asperities due to electrode surfaces, pinholes, and nonuniformities in organic layer thickness, for example.
Of particular concern for display operation is the defect caused by a shorted electroluminescent diode 11. Referring back to FIG. 2a, it can be observed that a shorted electroluminescent diode 11 for a pixel 10 effectively connects current source 22 directly to ground when the corresponding row is scanned. When other rows are scanned, a shorted electroluminescent diode 11 effectively sets intermediate voltage Vi onto anode line 26. Because of this, the complete column of pixels 10 is blacked out during display operation. Whereas some number of dead pixels 10 can be tolerated in a viewed image, defects affecting an entire line, in general, are not acceptable. Thus, in practice, there is zero tolerance for shorted pixel defects over the entire area of OLED array 20.
FIGS. 2b, 2c, 2d, and 2e show how various configurations of OLED array 20 behave in response to a shorted diode condition. FIGS. 2b, 2c, and 2d show OLED array 20 where switches 28 are either open or closed (to ground), without connection to intermediate voltage Vi. Referring first to FIG. 2b, there is shown a small section of OLED array 20 having electroluminescent diodes 11a, 11b, 11c, and 11d at individually addressable pixels 10a, 10b, 10c, and 10d, respectively, in an arrangement of rows 44a and 44b and columns 42a and 42b. In the example of FIG. 2b, electroluminescent diode 11d is shorted, as indicated by a short 46. During row scanning, row 44a is enabled, while adjacent row 44b is disabled, as shown at respective switches 28. Current source 22 for a column 42a is ON to illuminate pixel 10a (by providing current through electroluminescent diode 11a) at the intersection of column 42a and row 44a. However, short 46 is at the position of pixel 10d for the next row 44b at a column 42b. Short 46 thus provides an unwanted current path to column 42b, through electroluminescent diode 11c. Depending on the amount of current flowing through short 46, electroluminescent diode 11c can illuminate, thereby being permanently ON for scanning all rows 44 in array 20. Even dim constant illumination of electroluminescent diode 11c would be undesirable. As FIG. 2c shows, when both current sources 22 are ON, pixel 10c would have the desired state. As FIG. 2d shows, when row 44b is scanned, and electroluminescent diode 11c is ON, short 46 would be effectively bypassed.
Referring to FIG. 2e, there is shown an OLED array 20 arrangement in which switch 28 is at intermediate voltage Vi until a row is scanned. With short 46 in the position shown, when switch 28 for row 44b connects to idle voltage Vi and when row 44a is scanned, or any other row except row 44b is scanned, column 42b is held at Vi. Because of this, column 42b is effectively disabled. It is instructive to observe that current source 22 is designed to provide current to only a single electroluminescent diode 11 at a time; meanwhile, intermediate voltage Vi is provided to a full row 44a, 44b. It would be unpractical to size current source 22 in each column 42a, 42b to compensate for the condition caused by short 46.
The likelihood of a fabrication defect increases dramatically as the display area increases. Assuming that the overall defect density for array 20 exhibits a Poisson distribution characteristic, then the probability that array 20 has zero defects is the yield Y and can be expressed in the equation (1):Y=e−DA  (1)where D is the shorting defect density per area and, for a shorted diode 11, A is the full area of array 20.
The exponential scaling impact of defect density D and area A is particularly significant. For example, for a reasonable defect density D of 0.01 per cm and an area A of 0.5 square meter, the yield Y is as follows:Y=2×10−22.In other words, chances for a good display yield with a very large passive matrix OLED display are effectively nil. Only a dramatic reduction of factors D and A in the exponent of equation (1) can permit a reasonable yield for OLED arrays.
Active matrix OLED design provides one solution to this defect-related performance problem. In an active matrix OLED array, each individual OLED pixel 10 can be independently addressed, using an arrangement of thin-film transistors (TFTs) and storage capacitors. Active matrix display circuitry is disclosed in U.S. Pat. No. 6,392,617 to Gleason and U.S. Pat. No. 6,433,485 to Tai et al., for example. With an active matrix configuration, a shorted diode defect at any one OLED pixel 10 does not impact other OLED pixels 10. However, as is noted above, active matrix OLED array design is considerably more complex, requiring a number of additional support components for each OLED pixel 10.
U.S. Pat. No. 6,605,903 to Swallow discloses, as an alternative passive matrix approach, an array having sections that can be selectively activated or deactivated to compensate for OLED pixel 10 defects. In the OLED array of U.S. Pat. No. 6,605,903, each column has two separate sections, either of which can be activated or deactivated in the event of a shorted diode. While this approach can mitigate defect problems, the array requires a considerable number of additional components, many of which would not be used. Moreover, defects occurring after manufacture, and testing would still have a negative effect on display performance.
Although clearly not directed to an OLED array used for addressable image display, one solution proposed for large-scale OLED cells or modules used in room lighting and signage applications, outlined in U.S. Patent Application Publication 2002/0190661 A1 to Duggal et al., is of some interest. U.S. Patent Application Publication 2002/0190661 A1 discloses a serial connection of multiple, large area OLED modules directly to an AC power source. Each OLED cell or module is a single diode, having an emissive surface that is at least a few square centimeters in area. OLED cells are connected in series fashion, with the anode of one OLED cell connected to the cathode of the previous one, for example. Advantageously for the lighting and signage lettering uses described in U.S. Patent Application Publication 2002/0190661 A1, this solution permits OLED devices to be used with alternating current at line voltage (nominally at 120 VAC, 60 Hz), so that a separate DC power supply is not required. Series-connected OLED cells are arranged to illuminate during each half cycle of AC current. In a paper entitled “Fault-tolerant, scalable organic light-emitting device architecture” in Applied Physics Letters, Vol. 82, Number 16, 21 Apr. 2003, this type of series connection for large area OLED cells for illumination applications is also disclosed and further discussed with reference to the impact of faults on other OLED devices in the series. Not surprisingly, a shorted OLED cell diode in the series causes a corresponding increase in brightness among other OLED cells in the same series. However, the straightforward series connection described does have advantages over more complex fault response mechanisms.
Thus, while there have been a few solutions proposed for limiting or minimizing the impact of a faulted OLED on other nearby OLEDs, none of these solutions is particularly well suited for use with a passive matrix OLED array used in imaging display applications, where each OLED electroluminescent diode 11 serves as one individually addressable pixel 10 for forming an image. The active matrix designs disclosed in U.S. Pat. Nos. 6,392,617 and 6,433,485 add considerable complexity, as does the dual-column solution disclosed in U.S. Pat. No. 6,605,903. The solution proposed in U.S. Patent Application Publication 2002/0190661 A1 applies for discrete, modular OLED lighting devices that are used as banks of large-scale illuminators, rather than for OLED arrays where each individually addressable OLED electroluminescent diode 11 serves as one pixel 10 for forming an image. In considering any practical solution, it can be appreciated that there are benefits in maintaining the low cost and relative simplicity of the passive matrix OLED array design, as is shown in the schematic diagram of FIG. 2.
Thus, it can be seen that there is a need for an OLED array apparatus providing multiple individually addressable pixels and a method that provides a degree of tolerance to short conditions without adding substantial fabrication complexity or requiring complex support circuitry.